Computer simulation of surface-point defects interaction in hcp metals

Abstract

The static relaxation technique is used to analyze the structure of α-Zr and α-Ti perfect surfaces with different orientations as well as their interaction with vacancies and homologous adatoms. Atomic interactions are modeled by many-body potentials of the embedded atom type. Energies and (vibrational) entropies of formation and migration are obtained by minimizing the crystal energy and by further diagonalization of the force constant matrices, respectively. Entropies are calculated at the high temperature limit, considering the atoms as independent oscillators. The more costly coupled oscillators approach is also employed in order to assess the stability of some defect configurations and to envisage reaction coordinates. Relevant results are the following: (a) a lower surface density implies lower formation energies for adatoms and vacancies, but at the same time higher adatom migration energies; (b) surfaces act as sinks, absorbing vacancies and requiring high energies for their reemission; (c) vacancies compete with adatoms as surface diffusion mechanism, the corresponding activation energies being smaller than those for self-diffusion along symmetric grain boundaries and both in turn, smaller than those required for bulk diffusion.